One of the first events associated with pathogen infection in plants is the "oxidative burst , in which free radicals such as superoxide and hydrogen peroxide are synthesized at the site of infection. These reactive oxygen species (ROS) have been implicated by a number of studies to play a variety of roles in plant defense. However, free radicals can also inflict structural damage on cells, and their production must therefore be tightly controlled. The objectives of this proposal are to more critically assess and build upon existing models as to the roles of ROS in plant defense and to determine which antioxidant(s) is important for limiting the damage incurred from the pathogen-induced oxidative burst. This will be done by characterizing the interactions between Arabidopsis mutants that are deficient in detoxifying ROS and well-characterized pathogens of this plant and by isolating a novel class of mutants unable to produce free radicals in response to pathogens.